1. Field of the Invention
This invention relates to a parts mounting apparatus wherein electronic or other parts are mounted on other parts or members such as circuit substrates by mounting heads provided on a rotating carrier, and more particularly, to such a parts mounting apparatus wherein each of the mounting heads is provided with part sucking and mounting nozzles each switched between negative and positive pressure states for sucking the part and releasing and thereby mounting the same.
2. Description of the Prior Art
The assignees of the present application have jointly developed a parts mounting apparatus of the above-described type in which the parts mounting work can be performed at a high speed by continuous rotation of a rotating drum or carrier, and applied for patent on the invention on Mar. 26, 1991 in Japan. This Japanese application was published under the publication number of 3-274800. This developed apparatus comprises a rotating carrier driven by a motor, a feed station provided along a periphery of the rotating carrier for feeding parts to be mounted, a mounting station where other parts or members on which the parts fed at the feed station are to be mounted are disposed, and a plurality of mounting heads provided on the peripheral side of the rotating carrier and each having a plurality of suction nozzles sucking a part. Each mounting head alternately repeatedly performs picking up or sucking the parts at the feed station and placing or mounting them on the members at the mounting station with rotation of the rotating carrier. In this case, each mounting head performs a rotative movement about an axis extending radially with respect to the rotating carrier in synchronism with the same. This rotative movement of each mounting head is referred to as "cycloid movement." The above-mentioned sucking and mounting of the parts are performed at bottom dead points of the cycloid movement of each mounting head, at which points the mounting head is substantially stationary for about 4 milliseconds. That is, each of the sucking and the mounting of the parts needs to be performed in a short time period of about 4 milliseconds for which the mounting head is maintained at the bottom dead point of the cycloid movement.
Each suction nozzle is provided with a switching valve for switching the nozzle between positive and negative pressure states when the part is sucked and mounted. An encoder is provided for detecting a rotational position of the rotating carrier. A switching timing of each switching valve is based on outputs of the encoder.
An electromagnetic valve is used as the above-described switching valve. Several milliseconds or several ten milliseconds are required for a response time period of the switching valve which is a time period from the start of the switching to its completion. Accordingly, in the prior art, the switching of the valve is started based on the output of the encoder when the nozzle of the mounting head reaches a predetermined position before the bottom dead point, and the switching is completed when the nozzle reaches the bottom dead point.
If the above-described switching timing of the valve is shifted for some reason or other, the part is thrown out of the nozzle before or after it reaches the bottom dead point. Thus, the shift in the switching timing of the switching valve reduces accuracy in the mounting of the parts on the substrates. Experiments made by the inventors show that the shift of the mounting nozzle needs to be confined to a range of .+-.2 milliseconds.
However, the rotational speed of the rotating carrier of the above-described parts mounting apparatus slightly changes depending upon the size of the load. Furthermore, a case is considered where the rotational speed of the rotating carrier is positively adjusted. In such a case, when the switching valve is controlled based on the rotational position of the rotating carrier, the shift in the timings of sucking and the mounting of the parts sometimes exceeds the range of .+-.2 milliseconds.